The shower valve in the kids' bathroom gives a kind of double knocking sound when water is turned on or off in the house, particularly when the clothes washer or dishwasher do their on-off thing. This happened several years ago. I tried installing an air column here, a knock arrestor there, but nothing helped until we redid that bathroom and put in new shower hardware, which cured it completely. Now the knocking has returned, and I assume we will need to break into the wall and replace the shower valve again. The second unit was a different make and model than the first. Can anyone explain to me what went wrong with these valves, and how we might avoid it in the future? We do have hard water, if that could be a factor.